1. Technical Field
Example embodiments of the present invention relate in general to uplink demodulation reference signal (DM-RS) transmission, and more particularly, to a transmission method of multiplexing data and a DM-RS in one orthogonal frequency division multiplexing (OFDM) symbol while maintaining a low cubic metric (CM) and enabling multi-layer transmission of the multiplexed data and DM-RS in a digital Fourier transform (DFT)-spread (S)-OFDM technique, which is an uplink access scheme of 3rd Generation Partnership Project (3GPP) long term evolution (LTE)-advanced.
2. Related Art
The DFT-S-OFDM technique can efficiently make use of a power amplifier due to an excellent characteristic of a low CM or an equivalently low peak-to-average power ratio (PAPR). This becomes a significant advantage when coverage and the price of a power amplifier are issues.
To be specific, DFT-S-OFDM has a similar structure to OFDM, making it robust against a multipath channel, and also solves a problem of the conventional OFDM that a PAPR increases due to an inverse fast Fourier transform (IFFT) operation. In this way, DFT-S-OFDM reduces a PAPR of a final transmission signal by about 2 to 3 dB, thereby enabling a more efficient power amplifier (PA) to be used in a terminal. In the structure of single carrier frequency division multiple access (SC-FDMA), a load of a terminal is reduced as much as possible, and the reduced load is shifted to a base station.
Due to such an advantage, DFT-S-OFDM has been adapted as an uplink transmission scheme of a 3GPP LTE system, and is equivalently referred to as SC-FDMA.
Also, in 3GPP LTE-advanced, DFT-S-OFDM was considered an uplink transmission scheme and determined in early 2009 to be modified and applied as a clustered DFT-S-OFDM scheme, which allows non-contiguous resource allocation according to carrier aggregation considered in LTE-advanced.
Meanwhile, in most communication systems, a reference or pilot signal is transmitted from a transmission end to a reception end according to a predetermined rule in order to estimate a channel of the reception end, and so on. In 3GPP LTE, one OFDM symbol is used per slot to transmit a reference signal. This is intended to prevent a CM and PAPR from increasing due to characteristics of a DFT-S-OFDM signal when one OFDM symbol (which means a DFT-S-OFDM symbol but will be referred to as an OFDM symbol for convenience herein).
Meanwhile, LTE-advanced should provide a basic communication service even in a higher frequency spectrum and at a higher mobile velocity than LTE. For example, LTE-advanced requires support for a high mobile velocity of 350 km/h (even up to 500 km/h), which is the same as the velocity of a bullet train.
For this reason, LTE-advanced results in a higher Doppler spread environment than that considered in an LTE system, and it is difficult for the RS structure of a conventional LTE system to have satisfactory performance. To be specific, in the case of LTE, a length of each OFDM symbol is 66.67 μs, which is sufficiently shorter than a channel coherence time in a general environment. However, when a Doppler frequency caused by a high mobile velocity and a high frequency spectrum of a terminal, which are intended by LTE-advanced, is high, a fading channel may drastically vary even in one slot period (0.5 ms).
In addition, the current specification of LTE specifies only uplink DM-RS transmission in single-layer transceiving, and does not propose an uplink multi-layer (spatial multiplexing) transmission method to be introduced in LTE-advanced.